Origin of enhanced passivity of Cr–Fe–Co–Ni–Mo multi-principal element alloy surfaces

• Published in: Npj Materials degradation Vol. 7; no. 1; pp. 13 - 10
• Main Authors: Wang, Xueying, Mercier, Dimitri, Zanna, Sandrine, Seyeux, Antoine, Perriere, Loïc, Laurent-Brocq, Mathilde, Guillot, Ivan, Maurice, Vincent, Marcus, Philippe
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 16-02-2023; Nature Publishing Group; Nature Research; Nature Portfolio
• Subjects: 639/301/930/12; 639/638/542/971; Alloying elements; Alloys; Barrier layers; Chemical Sciences; Chemistry and Materials Science; Chloride resistance; Cobalt; Corrosion and Coatings; Corrosion resistance; Electrochemistry; Hydroxides; Ions; Iron; Localized corrosion; Material chemistry; Materials Science; Molybdenum; Nickel base alloys; Oxide coatings; Passivity; Photoelectrons; Secondary ion mass spectrometry; Structural Materials; Surface analysis (chemical); Tribology; Trivalent chromium; Passive films; Multi-principal element alloys; Surface analysis; Corrosion Protection
• ISSN: 2397-2106; 2397-2106
• DOI: 10.1038/s41529-023-00330-z

Surface analysis by X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry was applied to investigate the origin of the enhanced surface passivity and resistance to a chloride-induced breakdown provided by the protective ultrathin oxide films formed on Cr–Fe–Co–Ni–Mo single-phase fcc multi-principal element alloys. A bilayer structure of the oxide films is observed with the inner barrier layer mostly constituted of Cr(III) oxide and the outer layer enriched in Cr(III) hydroxides and Mo(IV,VI) oxides. The Mo(VI) and Mo(IV) species are mainly located in the outer and inner parts of the outer layer, respectively. Anodic passivation promotes mainly the growth of the inner layer on the alloy of higher Cr bulk content and the outer layer on the alloy of higher Mo bulk content. Passivation also promotes the enrichment of Cr(III) hydroxide and Mo(IV) and Mo(VI) oxides in the outer layer. Depth distribution analysis suggests that the ultra-thin protective inner barrier contains Cr(III)-depleted heterogeneities acting as weak sites for chloride attack, which are reinforced by the Mo(IV) oxide species concentrated close to the inner barrier layer. This elemental distribution provides an explanation for the reinforcement of the resistance to localized corrosion observed on these Cr–Fe–Co–Ni–Mo alloys.